Two recent technological advances have made large, flat-panel display devices a distinct practical possibility: (1) the preparation of liquid crystals exhibiting improved characteristics; and (2) the production of a surface layer of fine-grained polysilicon.
The development of liquid crystals has progressed to the point where miniature color television receiver sets have been produced from them and much larger information display panels are technically possible. Liquid crystals inherently demonstrate slow responses to electrical signals and, therefore, a "switch" to rapidly respond to an electrical stimulus is required to set up an active matrix display. The thin film transistor (TFT) performs that function.
It is well-known that a TFT can be fabricated from single crystal silicon; however, by the very nature of single crystal silicon, there is a limitation on the size of a TFT that can be fashioned therefrom. It has been discovered that a fine-grained polysilicon layer which has been deposited onto a substrate can be recrystallized to large-grained polysilicon by scanning with a heat source such as a laser. It has been observed that a TFT prepared from large-grained polysilicon evidences only slight deterioration in electrical characteristics when compared with those displayed by a TFT fabricated from single crystal silicon, and operates very satisfactorily in multiplexing a liquid crystal display.
A large-grained polysilicon active matrix display requires a substrate which is transparent, flat, smooth, inert, compatible with silicon, in terms of thermal expansion, and capable of withstanding processing temperatures of at least 850.degree. C. Accordingly, a glass will meet those requirements if it is free from alkali metal ions, it displays a linear coefficient of thermal expansion (25.degree.-300.degree. C.) of approximately 30-40.times.10.sup.-7 /.degree.C., and an annealing point of at least 850.degree. C. and, preferably, greater than 875.degree. C. In order to utilize the glass sheet forming method described in U.S. Pat. No. 3,338,696, the glass must exhibit a liquidus viscosity of at least 100,000 poises, preferably over 250,000 poises. Also, transparent, homogeneous glasses with annealing points in the vicinity of 900.degree. C. are very difficult to obtain by melting batches at temperatures no higher than about 1800.degree. C. 1800.degree. C. represents the practical limit for melting materials in contact with platinum-rhodium and most refractories used in an oxidizing atmosphere. Finally, to permit glass forming to be carried out utilizing conventional equipment and techniques, a liquidus temperature of no more than about 1400.degree. C. is highly preferred.
Therefore, the primary objective of the present invention is to prepare transparent, homogeneous glasses demonstrating a linear coefficient of thermal expansion (25.degree.-300.degree. C.) of about 30-40.times.10.sup.-7 /.degree.C., an annealing point of at least 850.degree. C., a liquidus temperature no higher than about 1400.degree. C., a liquidus viscosity of at least 100,000 poises, and which can be melted at temperatures not exceeding about 1800.degree. C.